1. Field of the Invention
The present invention relates to a control apparatus for image blur correction, which is applied to image blur correction devices for correcting an image blur occurring in optical equipment such as cameras and the like.
2. Related Background Art
In the cameras which are available presently, the important works for photography, including determination of exposure, focusing, etc., all are automated, and thus even those inexperienced in manipulation of camera have only a minimal possibility of causing a failure of photography.
In recent years research also has been conducted on systems for preventing hand vibration exerted on the camera and there are few factors to induce a photographer""s mistake in photography.
A system for preventing the hand vibration will be described below briefly.
The hand vibration of camera during photography is normally vibration in the frequency range of 1 Hz to 10 Hz, and a basic idea for permitting a photograph without an image blur to be taken even with such hand vibration upon release of the shutter is to detect vibration of the camera due to the above hand vibration and displace a correction lens according to a detected value thereof. For taking a photograph without an image blur even under the camera vibration, therefore, the first requirement is to accurately detect the vibration of the camera and the second requirement is to correct variation of the optical axis due to the hand vibration.
The detection of this vibration (camera vibration) can be implemented theoretically by providing the camera with a vibration detection device which is comprised of a vibration detection sensor for detecting acceleration, angular acceleration, angular velocity, angular displacement, or the like, and a calculation portion for calculating an output therefrom in order to correct the camera vibration on necessary occasions. Then the image blur is restrained by driving a correction means which decenters the photographing optical axis, based on this detection information.
FIG. 30 is a perspective view of appearance of a compact camera having the blur prevention system, which has the function of effecting the vibration correction for vertical vibration and horizontal vibration of camera indicated by arrows 42p, 42y with respect to the optical axis 41.
In the camera body 43, reference symbol 43a designates a shutter release button, 43b a mode dial (including the main switch), 43c a retractable flash (strobe), and 43d a finder port.
FIG. 31 is a perspective view to show the internal structure of the camera illustrated in FIG. 30. Reference numeral 44 denotes the camera body, 51 the correction means, 52 a correction lens, and 53 a support frame which is arranged to freely drive the correction lens 52 in directions 58p, 58y in the figure to correct the vibration in the directions of the arrows 42p, 42y of FIG. 30 and which will be detailed hereinafter. Reference symbols 45p, 45y represent vibration detection devices such as angular velocity sensors, angular acceleration sensors, or the like, which detect the vibration about the arrows 46p, 46y, respectively.
An output from each of the vibration detection devices 45p, 45y is converted into a drive target value of the correction means 51 through a calculation device 47p or 47y, described hereinafter, and the target value is entered into a corresponding coil of the correction means 51 to effect the blur correction. Reference symbol 54 indicates a base plate, 56p and 56y permanent magnets, and 510p and 510y coils.
FIG. 32 is a block diagram to show the details of the above-stated calculation devices 47p, 47y. Since these devices have like structure, the figure will be explained using only the calculation device 47p. 
The calculation device 47p is composed of a DC cut filter 48p, a low pass filter 49p, an analog-to-digital conversion circuit (hereinafter referred to as an A/D conversion circuit) 410p, a driving device 419p, and a camera microcomputer 411 indicated by a dashed line, which are enclosed in a chain line. The camera microcomputer 411 is composed of a memory circuit 412p, a differential circuit 413p, a DC cut filter 414p, an integration circuit 415p, a memory circuit 416p, a differential circuit 417p, and a PWM duty variation circuit 418p. 
In this case, the vibration detection device 45p is a vibration gyro which detects the angular velocity of vibration of the camera. The vibration gyro is driven in synchronization with on of the main switch of the camera to start detection of the angular velocity of vibration exerted on the camera.
An output signal of the vibration detection device 45p is supplied to the DC cut filter 48p constructed as an analog circuit, which cuts off the DC bias component superimposed on the output signal. This DC cut filter 48p has such a frequency characteristic as to cut the signal at the frequencies of not more than 0.1 Hz and does not affect the frequency band of the hand vibration ranging from 1 Hz to 10 Hz on the camera. This characteristic to cut the component at and below 0.1 Hz, however, poses a problem that approximately ten seconds are necessary for completely having cut the DC component since input of the vibration signal from the vibration detection device 45p. For this reason, the time constant of the DC cut filter 48p is set to a small value (for example, such a characteristic as to cut the signal at the frequencies of not more than 10 Hz), for example, up to 0.1 second after on of the main switch of the camera, whereby the DC component is cut off in the short time of about 0.1 second. Thereafter, the time constant is increased to a large value (such a characteristic as to cut the signal only at the frequencies of not more than 0.1 Hz) in order to prevent the DC cut filter 48p from degrading the angular velocity signal of vibration.
An output signal from the DC cut filter 48p is supplied to the low pass filter 49p constructed as an analog circuit, which properly amplifies the output signal so as to match it with resolving power of the A/D conversion circuit 410p and which cuts noise of high frequencies superimposed on the angular velocity signal of vibration. This is for avoiding occurrence of a read error due to the noise in the angular velocity signal of vibration, in sampling by the A/D conversion circuit 410p when the angular velocity signal of vibration is entered into the camera microcomputer 411. The output signal from the low pass filter 49p is sampled by the A/D conversion circuit 410p to be read into the camera microcomputer 411.
Although the DC cut filter 48p has cut the DC bias component, the amplification thereafter by the low pass filter 49p again causes the DC bias component to be superimposed on the angular velocity signal of vibration. Therefore, the DC component has to be cut again inside the camera microcomputer 411.
The cut of the DC component is carried out, for example, by storing the angular velocity signal of vibration sampled 0.2 second after on of the main switch of the camera, in the memory circuit 412p and obtaining a difference between the thus stored value and the angular velocity signal of vibration by the differential circuit 413p. Since this operation permits only rough cut of DC (or since the angular velocity signal of vibration stored 0.2 second after on of the main switch of the camera also includes the actual hand vibration as well as the DC component), adequate DC cut is effected by the DC cut filter 414p constructed of a digital filter in the subsequent stage. The time constant of this DC cut filter 414p is also variable, similar to the analog DC cut filter 48p, and the time constant is gradually increased during the period of 0.2 second from 0.2 second after on of the main switch of the camera. Specifically, this DC cut filter 414p has such a filter characteristic as to cut the signal at the frequencies of not more than 10 Hz when 0.2 second has passed since on of the main switch, and thereafter the frequencies to be cut by the filter are lowered every 50 msec in the descending order of 5 Hz, 1 Hz, 0.5 Hz, and 0.2 Hz.
There are, however, cases in which it is not preferable to vary the time constant at the expense of time, because there is a high possibility of carrying out photography immediately after the photographer half depresses the release button 43a (to turn sw1 on) to initiate photometry and distance measurement during the above operation. In such cases the variation of the time constant is suspended on the way according to the photographing conditions. For example, in the case wherein the photographing shutter speed is found to be {fraction (1/60)} from the photometry result and the photographing focal length is 150 mm, because the accuracy of blur prevention does not have to be so high, the variation of time constant of the DC cut filter 414p is completed at the time when the characteristic of cutting the signal at the frequencies of not more than 0.5 Hz is achieved. (A variation amount of the time constant is controlled by the product of the shutter speed and the photographing focal length.) This can reduce the time for the variation of time constant, so as to give a higher priority to a shutter chance. It is a matter of course that in cases of faster shutter speed or shorter focal length, the variation of time constant is completed at the time when the characteristic of the DC cut filter 414p reaches the characteristic of cutting the signal at the frequencies of not more than 1 Hz and that in cases of slower shutter speed or longer focal length, photography is inhibited until the variation of time constant is accomplished up to the last.
The integration circuit 415p starts integrating the output signal of the DC cut filter 414p in response to the half depression (on of sw1) of the release button 43a of the camera to convert the angular velocity signal to an angle signal. The integration circuit 415p, however, does not start the integral operation until the variation of time constant is completed in the above-stated cases wherein the variation of time constant of the DC cut filter 414p is not complete yet. Although not illustrated in FIG. 32, the angle signal as an integral result is amplified properly according to information of focal length and object distance at that time to be converted so as to drive the correction means 51 by an appropriate amount according to the angle of vibration. (This correction has to be carried out, because the photographing optical system is changed upon zooming or focusing to vary a decentering amount of the optical axis against a driving amount of the correction means 51.)
When the release button 43a is depressed fully (to turn sw2 on), the correction means 51 starts being driven according to the angle signal of vibration. At this time, it is necessary to exercise care to avoid suddenly starting the blur correction operation of the correction means 51. The memory circuit 416p and differential circuit 417p are provided as countermeasures against it. The memory circuit 416p stores the vibration angle signal from the integration circuit 415p in synchronization with the full depression of the release button 43a (on of sw2). The differential circuit 417p gains a difference between the signal of the integration circuit 415p and the signal of the memory circuit 416p. Therefore, the two signal input into the differential circuit 417p are equal upon on of the switch sw2, so that the drive target signal from the differential circuit 417p to the correction means 51 is zero at that time. Thereafter, the output is increased continuously from zero. (The memory circuit 416p functions to define the integral signal upon on of the switch sw2 as the origin.) This prevents the correction means 51 from being driven suddenly.
The target value signal from the differential circuit 417p is supplied to the PWM duty variation circuit 418p. When voltage or current corresponding to the vibration angle is applied to the coil 510p (see FIG. 31) of the correction means 51, the correction lens 52 is driven corresponding to the vibration angle. A desired driving method is PWM driving in terms of power saving of driving power consumption of the correction means 51 and power of a driving transistor of the coil.
The PWM duty variation circuit 418p varies the coil driving duty according to the target value. For example, in the case of the PWM at the frequency of 20 kHz, the duty is xe2x80x9c0xe2x80x9d when the target value of the differential circuit 417p is xe2x80x9c2048xe2x80x9d; the duty xe2x80x9c100xe2x80x9d for the target value of xe2x80x9c4096xe2x80x9d; between these two target values duties are determined according to respective target values at equal intervals. The determination of duty is finely controlled, not only depending on the target value, but also depending on the photographing conditions of the camera at that time (the temperature, the posture of the camera, a state of power supply), so as to effect the blur correction with accuracy.
An output from the PWM duty variation circuit 418p is supplied to the known driving device 419p such as a PWM driver or the like, and an output of the driving device 419p is applied to the coil 510p (see FIG. 31) of the correction means 51 to perform the blur correction. The driving device 419p is turned on in synchronization with on of the switch sw2 and is turned off after completion of exposure on film. As long as the release button 43a is kept depressed half (with sw1 on) even after completion of exposure, the integration circuit 415p keeps on integrating, and the memory circuit 416p again stores a new integral output upon next on of the switch sw2.
When the half depression of the release button 43a is stopped, the integration circuit 415p terminates the integral of the output from the DC cut filter 414p and the integration circuit 415p is reset. The xe2x80x9cresetxe2x80x9d operation is to null all the information as a result of integration heretofore.
With off of the main switch the vibration detection device 45p is turned off to terminate the blur prevention sequence.
When the output signal of the integration circuit 415p becomes greater than a predetermined value, it is judged that panning of the camera was done and the time constant of the DC cut filter 414p is varied. For example, the characteristic of cutting the signal at the frequencies of not more than 0.2 Hz is varied to the characteristic of cutting the signal at the frequencies of not more than 1 Hz and the time constant is returned again to the original value in a predetermined time. This time constant variation amount is also controlled by the magnitude of the output of the integration circuit 415p. Namely, when the output signal exceeds a first threshold, the characteristic of the DC cut filter 414p is set to the characteristic of cutting the signal at the frequencies of not more than 0.5 Hz; when the output signal exceeds a second threshold, the characteristic is set to the characteristic of cutting the signal at the frequencies of not more than 1 Hz; when the output signal exceeds a third threshold, the characteristic is set to the characteristic of cutting the signal at the frequencies of not more than 5 Hz.
When the output of the integration circuit 415p becomes very large, the integration circuit 415p is reset once in order to prevent saturation (overflow) in calculation.
In FIG. 32, the DC cut filter 414p is constructed to be activated 0.2 second after on of the main switch, but, without having to be limited to this, it may also be constructed to be activated with half depression of the release button 43a. In this case the integration circuit 415p is activated upon completion of the variation of time constant of the DC cut filter.
Further, the integration circuit 415p is also constructed to be activated with the half depression of the release button 43a (on of sw1), but it may also be constructed to be activated with the full depression (on of sw2) of the release button 43a. In this case, the memory circuit 416p and the differential circuit 417p can be excluded.
In FIG. 32 the DC cut filter 48p and the low pass filter 49p are set in the calculation device 47p, but it is needless to mention that these may be installed in the vibration detection device 45p. 
FIGS. 33, 34A, 34B and 35 are drawings to show the details of the correction means 51. More specifically, FIG. 33 is a front elevation of the correction means 51, FIG. 34A a side view thereof from the direction of an arrow 34A of FIG. 33, FIG. 34B a sectional view thereof along a line 34Bxe2x80x9434B of FIG. 33, and FIG. 35 a perspective view of the correction means 51.
In FIG. 33, the correction lens 52 is fixed to the support frame 53. (As illustrated in FIG. 34B, this correction lens 52 is composed of two lenses 52a, 52b fixed to the support frame 53, and a lens 52c fixed to the base plate 54 and thus composes a lens group of the photographing optical system.)
A yoke 55 of a ferromagnetic material is attached to the support frame 53 and permanent magnets 56p, 56y of neodymium or the like are fixed by attraction to the back surface of the yoke 55 in the same figure (as indicated by hidden outlines). Three pins 53a radially extending out of the support frame 53 are fit in respective elongate holes 54a provided in side walls 54b of the base plate 54.
As illustrated in FIGS. 34A and 35, each pair of pin 53a and elongate hole 54a engage with each other without play in the direction of the optical axis 57 of the correction lens 52, but the elongate hole 54a extends in the direction perpendicular to the optical axis 57. Therefore, the support frame 53 is restrained from moving in the direction of the optical axis 57 relative to the base plate 54 but can freely move in the plane perpendicular to the optical axis (along arrows 58p, 58y, 58r). However, the support frame 53 is elastically restrained from moving in each direction (58p, 58y, 58r), because tension springs 59 are stretched between hooks 53b on the support frame 53 and hooks 54c on the base plate as illustrated in FIG. 33.
Coils 510p, 510y (indicated by hidden outlines in part) are attached to the base plate 54 opposite to the permanent magnets 56p, 56y. The placement of the yoke 55, permanent magnet 56p, and coil 510p is as illustrated in FIG. 34B (and the permanent magnet 56y and the coil 510y are also arranged in the same placement). The support frame 53 is driven in the directions of the arrow 58p with supply of current to the coil 510p, and the support frame 53 is driven in the directions of the arrow 58y with supply of current to the coil 510y. 
A driving amount of the support frame is determined by a balance between the spring constant of the tension springs 59 and the thrust generated from the relation between the coils 510p, 510y and the permanent magnets 56p, 56y in each direction. Namely, a decentering amount of the correction lens 52 can be controlled based on current amperes supplied to the coils 510p, 510y. 
As explained with FIG. 32, the DC bias component superimposed on the signal is cut off from the signal of the vibration detection device 45p (45y) by the DC cut filter 48p constructed as an analog circuit. This DC cut filter 48p is constructed in structure composed of an operational amplifier 420p, a capacitor 421p, resistors 422p, 423p, and a switch 424p, as illustrated in FIG. 36. (The DC cut filter of the vibration detection device 45y is also constructed in like structure.) For setting the characteristic of this DC cut filter 48p to the characteristic of cutting the frequencies of not more than 0.1 Hz, for example, the capacitor 421p is one with the capacitance of 10 xcexcF and the resistor 422p is one with the resistance of 160 kxcexa9.
Let us suppose here that the resistance of the resistor 423p is 1.6 kxcexa9, for example. Then this DC cut filter 48p cuts the frequencies of not more than 10 Hz when the switch 424p is closed. When the switch 424p is opened, the DC cut filter 48p cuts the frequencies of not more than 0.1 Hz. Therefore, the DC component can be cut off in the early stage by closing the switch 424p, for example, during the period of 0.1 second after on of the main switch of the camera, as described previously.
Incidentally, the capacitance 421p used in the circuit configuration of FIG. 36 is the one with the large capacitance, 10 xcexcF, which posed the problems that the circuit became considerably large and that the cost also became high. This configuration of the DC cut filter 48p also poses a further problem of degrading the accuracy of blur prevention. This will be explained referring to FIGS. 37A and 37B.
FIGS. 37A and 37B are conceptual diagrams to show the frequency characteristics of the DC cut filter 48p of FIG. 36, in which a line segment 425 indicates the ratio (gain) of output signal to input signal of the DC cut filter 48p and in which a line segment 426 indicates the phase of output signal to input signal similarly.
Referring to the line segment 425, the gain decreases in the frequency range lower than the frequency of 0.1 Hz and it is seen from this fact that signal outputs below this frequency are attenuated, so as to achieve the DC cut characteristic.
For carrying out the blur prevention with accuracy, it is necessary to enter the signal of the vibration detection device into the correction means with minimizing phase shift, but it is seen from the line segment 426 that the phase advances, particularly, in the lower frequency range than the main band of the hand vibration ranging from 1 to 10 Hz, so as to fail to carry out the blur prevention with accuracy.
The blur prevention accuracy can be improved by changing the DC cut filter now cutting the frequencies of not more than 0.1 Hz, to the characteristic of cutting the frequencies below 0.01 Hz, for example. This change, however, requires increase of the capacitance of the capacitor 421p, for example, to 100 xcexcF (or needs to increase the resistance of the resistor 422p to 1.6 Mxcexa9), which is not preferable from aspects of circuit scale and noise.
As described above, the DC cut filters heretofore had the problems of requiring the large capacitor, being unsuitable for reduction of size and cost, and degrading the blur prevention accuracy.
One aspect of the invention is a control apparatus for image blur correction, which is applied to an image blur correction device, said control apparatus comprising:
a microcomputer which forms a driving signal for driving the image blur correction device in accordance with a signal corresponding to an output of a vibration detection sensor;
an offset signal removing circuit (1) which removes an offset signal from the output of the vibration detection sensor before the signal corresponding to the output of the vibration detection sensor is entered into the microcomputer and (2) which supplies a signal resulting from the removal of the offset signal, as the signal used for the formation of said driving signal by said microcomputer, to said microcomputer, (3) a removal offset value of said offset signal removing circuit being set variably;
a removal offset value setting circuit which sets said removal offset value for said offset signal removing circuit and which thereafter makes said offset signal removing circuit carry out a removal operation based on the removal offset value thus set; and
instruction means which gives an instruction for carrying out a setting operation of said removal offset value, to said removal offset value setting circuit, said instruction means giving the instruction for carrying out the setting operation of the removal offset value even in a state in which said offset signal removing circuit has already executed said offset signal removal operation and is entering said signal resulting from the removal of the offset signal into said microcomputer.
This allows the setting operation of the removal offset value to be carried out again in the state in which the offset signal removing circuit has executed the offset signal removal operation and is entering the signal resulting from the removal of the offset signal into the microcomputer, thereby realizing more accurate image blur correction control.